A high level of reliability and the ability to quickly diagnose a hardware problem are decisive factors in the successful operation of fiberoptic networks. The measurement of optical power, in the fiber is used to maintain and troubleshoot the network. The complex and flexible architectures of WDM networks generate a wide variety of signal combinations in the same fiber. To characterise the behaviour of such networks, an optical power detector is typically utilized having a wide dynamic range. To perform a measurement, a fraction of light energy from the fiber is coupled into the detector. By keeping the amount of diverted light small, signal losses are reduced. Therefore, it is advantageous to use a detector that is sensitive to low-level signals. Optical power detectors usually include photodiodes, in combination with transimpedance amplifiers. They transform the optical power into an analog voltage signal.
To increase the dynamic range of these detectors, logarithmic transimpedance amplifiers with a p-n junction can be used. However, photodiodes with logarithmic transimpedance amplifiers are typically not widely used due to several significant disadvantages. One such disadvantage is the limitation of sensitivity of the detector to low optical power due to the influence of the dark current of the photodiode. The dark current is highly temperature dependant and, thus, difficult to correct for with temperature variations. Additionally, the leakage current through the detector is temperature and humidity dependant and its influence also limits the sensitivity of the detector. Lastly, the logarithmic transform characteristic of the amplifier is temperature dependant because of the physics of the p-n junction. Accordingly, as temperature varies, the above mentioned phenomena contribute to the limitation of the detector.
Another significant limitation of logarithmic transimpedance amplifiers is limited frequency response. The cause of it is a combination of a very high equivalent dynamic resistance of the p-n junction at the bottom of the dynamic range of the amplifier, capacitance of the junction and stray capacitance of the circuit board.
In accordance with an aspect of the present invention, an apparatus for a wide dynamic range photodetector is provided. According to an aspect of the present invention, an optical power detector including a photodiode and a logarithmic transimpedance amplifier is mounted inside a hermetically sealed package in such a way that the photodiode and the logarithmic transimpedance amplifier are located on the same thermally conductive substrate. A control apparatus, such as a thermal electric cooler, thermistor and electronic control circuit, maintains the constant temperature of the substrate and components mounted thereon. A benefit of the present invention is that stabilization of photodiode dark current, logarithmic transimpedance amplifier transfer characteristics, and circuit leakage currents, are thereby achieved.
The second benefit of the invention is that due to the use of hybrid microelectronic assembly technology inside the package the stray capacitance is kept down providing higher frequency response.
In a further aspect of the present invention, a wide dynamic range optical power detector is described that includes a photodiode for receiving an optical signal, a logarithmic transimpedance amplifier with a p-n junction device as a logarithmic element, coupled to the photoidiode and a thermally conductive substrate having the above mentioned devices mounted thereon such that they are maintained at essentially the same constant temperature independent of the environment temperature.
More specifically, the optical power detector includes a thermal electric cooler and a temperature measurement device, both coupled to the substrate, and a temperature control circuit, coupled to the temperature measurement device. The temperature control circuit receives temperature measurements of the substrate and responsively sends controlling signals to the thermal electric cooler in order to maintain a constant temperature.